The present invention relates to a magnetoresistive effect (MR) thin-film magnetic head that is applicable to a hard disk drive (HDD) apparatus and provided with a tunnel magnetoresistive effect (TMR) element or a current perpendicular to the plane giant magnetoresistive effect (CPP-GMR) element, in which a current flows in a direction perpendicular to surfaces of layers.
Recently, in order to satisfy the demand for higher recording density in an HDD apparatus, higher sensitivity and larger output of a thin-film magnetic head are required. A TMR element and a CPP-GMR element meet these requirements and are beginning to receive attention. The TMR element, disclosed in Japanese patent publication No. 04103014A for example, utilizes a ferromagnetic tunnel effect and has a multi-layered structure including a lower ferromagnetic thin-film layer, a tunnel barrier layer and an upper ferromagnetic thin-film layer. The CPP-GMR element is one type of GMR element of a multi-layered structure including a lower ferromagnetic thin-film layer, a nonmagnetic metal layer and an upper ferromagnetic thin-film layer. In the CPP-GMR element, however, a current flows in a direction perpendicular to the surfaces of laminated layers. Such CPP-GMR element is disclosed in, for example, W. P. Pratt, Jr. et al., xe2x80x9cPerpendicular Giant Magnetoresistance of Ag/Co Multilayer,xe2x80x9d PHYSICAL REVIEW LETTERS, Vol. 66, No. 23, pp.3060-3063, June 1991.
These elements not only offer MR ratios several times greater than that of a general GMR element such as CIP (Current-In-Plane)-GMR element in which a current flows along the surface of layers, but also implements narrow gaps between layers without difficulty. The terms xe2x80x9clowerxe2x80x9d in xe2x80x9clower ferromagnetic thin-film layerxe2x80x9d and xe2x80x9cupperxe2x80x9d in xe2x80x9cupper ferromagnetic thin-film layerxe2x80x9d are selectively used depending on the position of the layer relative to the substrate. In general, a layer is xe2x80x9clowerxe2x80x9d if this layer is close to the substrate, and xe2x80x9cupperxe2x80x9d if the layer is away from the substrate.
FIG. 1 illustrates a CIP-GMR element with a conventional structure seen from an air bearing surface (ABS).
In the figure, reference numeral 10 denotes a lower shield layer, 11 denotes a lower shield gap layer made of an insulation material, 12 denotes a GMR multilayer consisting of a lower ferromagnetic thin-film layer (free layer)/a nonmagnetic metal layer/an upper ferromagnetic thin-film layer (pinned layer)/an anti-ferromagnetic thin-film layer, 13 denotes an upper shield gap layer formed of an insulation material, 14 denotes an upper shield layer, 15 denotes hard bias layers, and 16 denotes electrode layers, respectively.
A sense current flows in parallel to the surfaces of the layers of the GMR multilayer 12. The GMR multilayer 12 are insulated from the lower shield layer 10 by the lower shield gap layer 11, and from the upper shield layer 14 by the upper shield gap layer 13.
In order to narrow the gap of such a CIP-GMR element more, the lower and upper shield gap layers 11 and 13 are required to be formed of a very thin insulating material with a very high dielectric strength. However, such an insulating material is difficult to make and has been the bottleneck for providing a CIP-GMR element used in a high density HDD apparatus.
FIG. 2 illustrates a TMR element or a CPP-GMR element with a conventional structure, seen from the ABS.
In the figure, reference numeral 20 denotes a lower shield layer also serving as an electrode, 21 denotes a lower gap layer made of a metal material, which also serves as an electrode, 22 denotes a TMR layer with a multi-layered structure consisting of a lower ferromagnetic thin-film layer (free layer)/a tunnel barrier layer/an upper ferromagnetic thin-film layer (pinned layer)/an anti-ferromagnetic thin-film layer, or CPP-GMR layer with a multi-layered structure consisting of a lower ferromagnetic thin-film layer (free layer)/a nonmagnetic metal layer/an upper ferromagnetic thin-film layer (pinned layer)/an anti-ferromagnetic thin-film layer, 23 denotes an upper gap layer made of a metal material, which also serves as an electrode, 24 denotes an upper shield layer also serving as an electrode, 25 denotes hard bias layers, and 26 denotes an insulation gap layer made of an insulating material, respectively. Reference numeral 22a denotes extended parts of the lower ferromagnetic thin-film layer (free layer) extending from the TMR multilayer or the CPP-GMR multilayer to the hard bias layers 25 along the surfaces of layers of the TMR multilayer or the CPP-GMR multilayer.
The TMR element or CPP-GMR element is electrically connected between the lower shield layer 20 and the upper shield layer 24 so that a sense current flows in a direction perpendicular to the surfaces of the layers. Therefore, a narrow gap can be implemented without inviting dielectric breakdown of the gap layer. As a result, the line recording density can be greatly improved.
The important features required for an HDD apparatus are not only high recording density but also high data transfer rate. The transfer rate greatly relies on the rotational speed of a magnetic disk as well as the frequency characteristics of a write head and a read head.
FIG. 3 shows an equivalent circuit of the CIP-GMR element, and FIG. 4 shows an equivalent circuit of the TMR element or the CPP-GMR element.
As is apparent from FIG. 3, the CIP-GMR element has only an equivalent resistance RGMR of the GMR element across the output terminals and no other essential factor that may deteriorate its frequency characteristics. However, as shown in FIG. 4, the TMR element or the CPP-GMR element that utilizes the shield layers as the electrodes has not only an equivalent resistance RTMR of the TMR element or the CPP-GMR element across their output terminals but also a capacitance Cshield between the shield layers and a capacitance CTMR of the TMR element or the CPP-GMR element itself across their output terminals. These resistance RTMR and capacitances CTMR and Cshield form a low-pass filter causing serious deterioration of the frequency characteristics.
It is therefore an object of the present invention to provide an MR thin-film magnetic head having a TMR element or a CPP-GMR element for example, whereby the frequency characteristics of the MR thin-film magnetic head can be greatly improved.
According to the present invention, an MR thin-film magnetic head includes a lower shield layer, an MR multilayer laminated on the lower shield layer, in the MR multilayer, a current flowing in a direction perpendicular to surfaces of layers of the MR multilayer, an upper gap layer made of a nonmagnetic electrically conductive material and laminated on the MR multilayer, an insulation gap layer made of insulation material and formed between the lower shield layer and the upper gap layer, and an upper shield layer laminated on the upper gap layer. A thickness of the insulation gap layer is larger than that of the upper gap layer.
Also, according to the present invention, an MR thin-film magnetic head includes a lower shield layer, a lower gap layer made of a nonmagnetic electrically conductive material and laminated on the lower shield layer, an MR multilayer laminated on the lower gap layer, in the MR multilayer, a current flowing in a direction perpendicular to surfaces of layers of the MR multilayer, an upper gap layer made of a nonmagnetic electrically conductive material and laminated on the MR multilayer, an insulation gap layer made of an insulation material and formed between the lower shield layer and the upper gap layer, and an upper shield layer laminated on the upper gap layer. A thickness of the insulation gap layer is larger than that of the upper gap layer.
In case of CIP-GMR head, it is common sense to locate a CIP-GMR multilayer at the center of gap between lower and upper shield layers from the viewpoint of insulation strength. Contrary to this, in case of CPP-GMR head or TMR head in which a current flows in a direction perpendicular to surfaces of layers, there occurs no problem even if its conductive gap layer is thinned. Thus, according to the present invention, the thickness of the insulation gap layer is made larger than that of the upper gap layer so as to thicken the insulation gap layer. As a result, the capacitance Cshield between the lower shield layer and the upper shield layer decreases to extremely improve the frequency characteristics of the thin-film magnetic head.
FIG. 5 illustrates a head output versus frequency characteristic when the capacitance Cshield between the shield layers in the equivalent circuit of FIG. 4 is 6 pF, and FIG. 6 illustrates a head output versus frequency characteristic when the capacitance Cshield between the shield layers in the equivalent circuit of FIG. 4 is 1 pF. It is assumed that the capacitance CTMR of the TMR element or CPP-GMR element itself is 0.01 pF and the load connected across the output terminals is 10 Mxcexa9.
As will be understood from FIG. 5, when the capacitance Cshield between the shield layers is 6 pF, the cut-off frequency fc at which the output decreases by 3 dB decreases as the resistance RTMR increases. In order to achieve fc greater than 500 MHz, the resistance RTMR should be less than 50 xcexa9. This frequency of 500 MHz is an expected frequency to be used at a record density of about 100 Gbits/in2. For TMR or CPP-GMR elements with a recording density of 100 Gbits/in2 or more, it is very difficult to implement such a low resistance value.
Contrary to this, as shown in FIG. 6, when the capacitance Cshield between the shield layers is 1 pF, even if the resistance RTMR is higher than 300 xcexa9, the cut-off frequency fc can be fc greater than 500 MHz. For the resistance RTMR higher than 300 xcexa9 that is a sufficiently realizable value, the frequency characteristic of the thin-film magnetic head can be greatly improved by making the capacitance Cshield between the shield layers smaller. The capacitance CTMR of the TMR element or CPP-GMR element itself is much smaller than the capacitance Cshield between the shield layers (less than one tenth), and therefore can be of little or no problem.
In order that the thickness of the insulation gap layer becomes larger than that of the upper gap layer, it is preferred that a lower gap layer made of a nonmagnetic electrically conductive material and provided with a large thickness is formed under the MR multilayer.
In order that the thickness of the insulation gap layer becomes larger than that of the upper gap layer, it is also preferred that the MR multilayer has a large thickness. In this case, preferably, the MR multilayer has an anti-ferromagnetic layer with a large thickness.
It is further preferred that a ratio THG2/THG1 of a thickness THG2 of the upper gap layer and a thickness THG1 of the insulation gap layer is more than 1/5 to improve the output characteristics of the thin-film magnetic head.
The MR multilayer is preferably a TMR multilayer including a tunnel barrier layer and a pair of ferromagnetic thin-films between which the tunnel barrier is sandwiched, or a CPP-GMR multilayer including a nonmagnetic metal layer, and a pair of ferromagnetic thin-films between which the nonmagnetic metal layer is sandwiched.
Further objects and advantages of the present invention will be apparent from the following description of preferred embodiments of the invention as illustrated in the accompanying drawings.